Anodic films for high performance aluminum alloys

ABSTRACT

Anodic films that provide improved corrosion resistance to high performance aluminum alloys, and methods for forming the same, are described. According to some embodiments, the anodic films have a dense porous layer and a thickened barrier layer. The porous layer can act as a cosmetic portion of the anodic film and have pores that have a colorant infused therein. The thickened barrier layer can distribute defects within the anodic film associated with alloying elements of the high performance aluminum alloy in a larger non-porous film compared to conventional anodic films, thereby lessening the chance of corrosion inducing agents of reaching the high performance aluminum alloy. The anodic films have superior scratch and chemical resistance, as well as enhanced cosmetic aspects, well suited for consumer products, such as housings for electronic products.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 62/249,079, filed Oct. 30, 2015, and entitled “ANODIZED FILMS WITH PIGMENT COLORING,” which is incorporated herein by reference in its entirety and for all purposes.

Any publications, patents, and patent applications referred to in the instant specification are herein incorporated by reference in their entireties. To the extent that the publications, patents, or patent applications incorporated by reference contradict the disclosure contained in the instant specification, the instant specification is intended to supersede and/or take precedence over any such contradictory material.

FIELD

The described embodiments relate to anodized films with enhanced corrosion protection properties that are useful for protecting high performance aluminum alloys. Described are methods for forming anodic films that include structural features that reduce deleterious of defects within the anodic films, thereby increasing corrosion protection of an underlying alloy substrate.

BACKGROUND

Anodizing is an electrochemical process that thickens a naturally occurring protective oxide on a metal surface. An anodizing process involves converting part of a metal surface to an anodic film. Thus, an anodic film becomes an integral part of the metal surface. Due to its relative hardness, an anodic film can provide corrosion resistance and wear protection for an underlying metal. In addition, an anodic film can enhance a cosmetic appearance of a metal surface. For example, anodic films can have a porous microstructure that can be infused with dyes to impart a desired color to the anodic films.

When conventional anodizing methods are applied to some high performance aluminum alloys, however, certain types of defects can form within the anodic film. These defects can act as entry points for water or other corrosion-inducing agents to enter the anodic film and cause corrosion of the underlying metal substrate. What is needed therefore are improved methods for forming corrosion preventing and cosmetically appealing anodic films on high performance alloys.

SUMMARY

This paper describes various embodiments that relate to anodic films on high performance alloys, such as high strength aluminum alloys, and methods for forming the same. The anodic films can provide increased corrosion protection for the high performance alloys.

According to one embodiment, a method of anodizing an aluminum alloy substrate is described. The method includes forming a metal oxide film on the aluminum alloy substrate by anodizing the aluminum alloy substrate in a first electrolyte. The metal oxide film includes a porous layer and a barrier layer. The method also includes increasing a thickness layer of the barrier layer by anodizing the aluminum alloy substrate in a second electrolyte different than the first electrolyte. A final thickness of barrier layer ranges between about 30 nanometers to 500 about nanometers—in some cases, ranging between about 50 nanometers to about 500 nanometers. The porous layer includes pores having diameters ranging between about 10 nanometers to about 30 nanometers—in some cases, ranging between about 10 nanometers to about 20 nanometers. In some embodiments, the pores are defined by pore walls have thicknesses ranging between about 10 nanometers to about 30 nanometers.

According to another embodiment, an anodized part is described. The anodized part includes an aluminum alloy substrate and an anodic film disposed on the aluminum alloy substrate. The anodic film includes an exterior oxide layer having an outer surface corresponding to an outer surface of the anodized part. The exterior oxide layer includes pores having diameters ranging from about 10 nanometers to about 30 nanometers. The anodic film also includes a barrier layer positioned between the exterior oxide layer and the aluminum alloy substrate. A thickness of the barrier layer ranges between about 30 nanometers and 500 about nanometers—in some cases, ranging between about 50 nanometers to about 500 nanometers.

According to a further embodiment, an enclosure for an electronic device is described. The enclosure includes an aluminum alloy substrate having at least 4.0% by weight of zinc—in some cases, at least 5.4% by weight of zinc. The enclosure also includes an anodic coating disposed on the aluminum alloy substrate. The anodic coating includes an exterior oxide layer having sealed pores defined by pore walls. The sealed pores have diameters ranging between about 10 nanometers to about 30 nanometers—in some cases, ranging between about 10 nanometers and about 20 nanometers. The anodic coating also includes a barrier layer positioned between the exterior oxide layer and the substrate. A thickness of the barrier layer ranges between about 30 nanometers to 500 about nanometers—in some cases, ranging between about 50 nanometers to about 500 nanometers.

These and other embodiments will be described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements.

FIG. 1 shows perspective views of devices having metallic surfaces that can be protected using anodic films described herein.

FIG. 2 shows a cross section view of an anodized part illustrating how using a conventional anodizing process on high performance alloys can cause defects within an anodic film.

FIGS. 3A-3D show cross section views of an anodized part with enhanced corrosion and aesthetic characteristics, in accordance with some embodiments.

FIG. 4 shows a flowchart indicating a process for forming a metal oxide coating, in accordance with some embodiments.

FIGS. 5A and 5B show TEM cross section images of anodic films prior to a barrier layer thickening process, in accordance with some embodiments.

FIGS. 6A and 6B show TEM cross section images of the anodic films of FIGS. 5A and 5B after a barrier layer thickening process, in accordance with some embodiments.

FIGS. 7A and 7B show SEM cross section images of anodic films prior to and after a barrier layer thickening process, respectively, in accordance with some embodiments.

FIGS. 8-12 show aluminum alloy samples with and without a thickened barrier layer before and after a salt-spray test and an ocean water test, indicating the effectiveness of a thickened barrier layer for protecting an underlying substrate from corrosion.

DETAILED DESCRIPTION

Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims.

Described herein are processes for providing anodic films that provide superior corrosion protection and cosmetic qualities to high performance aluminum alloys. In particular embodiments, the anodic films have a dense exterior porous layer, which can correspond to an outer layer of the anodic film. The pore walls of the porous layer can be thicker than conventional anodic films, thereby providing a high hardness and high chemical resistivity. The porous layer can include pores that can hold colorants, such as dyes or pigments, thereby providing cosmetic qualities to the anodic film. The anodic films can also include a thickened non-porous barrier layer positioned beneath the porous layer. The dense porous layer and the thickened barrier layer can ameliorate corrosion susceptibility due to the presence of defects within the anodic film associated with certain alloying elements of high performance aluminum alloys. In these ways, the anodic films can provide a cosmetically appealing and high corrosion resistance coating to the underlying high performance aluminum alloy.

Methods for forming the anodic films can include using a first anodizing electrolyte to form a porous layer, and a second anodizing electrolyte to thicken an existing a non-porous barrier layer. In some embodiments, the first electrolyte includes oxalic acid or sulfuric acid, under conditions that can form a relatively dense and chemically resistant porous layer. The second electrolyte can include a non-dissolution chemical, such as borax or boric acid. The anodic film can be sealed using a sealing process to further increase its chemical resistance and corrosion resistance. The resultant anodic film can have a hardness of at least 200 HV and a corrosion resistance of about 312 hours using salt spray testing. In some embodiments, the anodic film is colorized using a dye or pigment. In some embodiments, a final color of the anodic film is determined by adjusting one or more of the first electrolyte, the thickness of the barrier layer, the smoothness of the barrier layer, or type of colorant infused within pores of the anodic film.

The present paper makes reference to anodizing of aluminum and aluminum alloy substrates. It should be understood, however, that the methods described herein may be applicable to any of a number of other suitable anodizable metal substrates, such as suitable alloys of titanium, zinc, magnesium, niobium, zirconium, hafnium, and tantalum, or suitable combinations thereof. As used herein, the terms anodized film, anodized coating, anodic oxide, anodic coating, anodic film, anodic layer, anodic coating, anodic oxide film, anodic oxide layer, anodic oxide coating, metal oxide film, metal oxide layer, metal oxide coating, oxide film, oxide layer, oxide coating etc. can be used interchangeably and can refer to suitable metal oxides, unless otherwise specified.

Methods described herein are well suited for providing cosmetically appealing surface finishes to consumer products. For example, the methods described herein can be used to form durable and cosmetically appealing finishes for housing for computers, portable electronic devices, wearable electronic devices, and electronic device accessories, such as those manufactured by Apple Inc., based in Cupertino, Calif.

These and other embodiments are discussed below with reference to FIGS. 1-12. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting.

The methods described herein can be used to form durable and cosmetically appealing coatings for metallic surfaces of consumer devices. FIG. 1 shows consumer products that can be manufactured using methods described herein. FIG. 1 includes portable phone 102, tablet computer 104, smart watch 106 and portable computer 108, which can each include housings that are made of metal or have metal sections. Aluminum alloys can be a choice metal material due to their light weight and ability to anodize and form a protective anodic oxide coating that protects the metal surfaces from scratches. The choice of aluminum alloy type will depend on desired physical and cosmetic characteristics. For example, some 6000 series aluminum alloys can provide excellent corrosion resistance and be anodized to form cosmetically appealing anodic oxide coatings. For example, some 6000 series aluminum alloys (e.g., some 6063 aluminum alloy substrates) can be anodized using type II anodizing (as defined by U.S. military specification MIL-A-8625), which involves anodizing in sulfuric acid based solution, to form relatively translucent and cosmetically appealing anodic oxide coating.

Some 2000 and 7000 series aluminum alloys are considered high performance aluminum alloys since they can have high mechanical strength. For this reason, it may be desirable to form housings for electronic devices using these high performance aluminum alloys. However, these high performance aluminum alloys can be more susceptible to corrosion due to the relatively high concentrations of certain alloying elements. Anodizing can help protect exposed surfaces of these high performance aluminum alloys. However, anodizing high performance aluminum alloys can result in anodic oxide coatings that include defects, thought to be related to some of these alloying elements. For example, some 2000 series aluminum alloys can have relatively large concentrations of copper, and some 7000 series aluminum alloys can have relatively large concentrations of zinc. These defects within the anodic oxide coatings can act as entry points for water or other corrosion inducing agents to penetrate the anodic oxide coatings and reach the underlying aluminum alloys substrates.

Described herein are improved techniques for providing improved anodic oxide coatings for high performance aluminum alloys that prevent or reduce the occurrence of such corrosion-related defects. Note that the methods can also be used for providing anodic oxide coatings for aluminum alloys that are not considered high performance, or other suitable anodizable substrates. For example, although some 2000 series and some 7000 series aluminum alloys may benefit from the anodic oxide coating described herein, 6000 series aluminum alloys (e.g., 6063 aluminum alloys) may also benefit from having the anodic oxide coating described herein over conventional anodic oxide coatings.

FIG. 2 shows a cross section view of a surface portion of anodized part 200. Part 200 includes metal substrate 202 and metal oxide coating 204. Metal substrate can be composed of a high strength aluminum alloy, which includes alloying elements that enhance the mechanical strength of metal substrate 202. Metal oxide coating 204 can be formed using an anodizing process whereby a surface portion of metal substrate 202 is converted to a corresponding metal oxide material 203 (e.g., aluminum oxide). Metal oxide coating 204 includes pores 206, formed during the anodizing process, defined by pore walls 205. The size of pores 206 can vary depending on the anodizing process conditions. For example, some type II anodizing processes can result in pores 206 having diameters of about 20 nanometers to about 30 nanometers. Metal oxide coating 204 includes porous layer 201 (defined by thickness 212) and a barrier layer 209 (defined by thickness 214), which corresponds to a generally non-porous portion of metal oxide coating 204 between metal substrate 202 and porous portion 201. Thickness 214 of barrier layer 209 is typically on the order of 10 nanometers to about 30 nanometers.

As shown, defects 207 can form within metal oxide coating 204. Defects 207 can correspond to inconsistencies within the structure of metal oxide material 203—in some cases defects 207 are in the form of cracks. Defects 207 can be associated with the type and amount of alloying elements within metal substrate 202. For example, defects 207 can be associated with relatively high concentrations of zinc or copper, which can be found in some 7000 series alloys and some 2000 series alloys, respectively. Defects 207 can be small, sometimes in the order of nanometers or tens of nanometers (e.g., as small as around 10 nm). However, some defects 207 are large enough to span thickness 214 of barrier layer 209. For example, defects 207 can connect with each other, thereby spanning thickness 214. In some cases, defects 207, such as cracks, can become bigger during manufacture process or service lifetime of anodized part 300. For example, thermal cycling can cause small crack defects to become larger. In this way, defects 207 can act as entry points for water or other corrosion inducing agents to reach metal substrate 202. For example, corrosion inducing agents can enter exterior surface 210 of metal oxide coating 204 via pores 206, pass through barrier layer 209 via defects 207 and reach metal substrate 202. In some cases, defects 207 can allow corrosion inducing agents to reach metal substrate 202 even if pores 206 are sealed using a hydrothermal sealing process. If metal substrate 202 is relatively susceptible to corrosion, such as some 7000 and 2000 series aluminum alloys, metal substrate 202 can corrode, thereby degrading the adhesion of metal oxide coating 204 and the integrity of anodized part 200.

Methods described herein involve forming metal oxide coatings that provide improved corrosion protection for high performance alloys. FIGS. 3A-3C illustrate cross section views of part 300 undergoing an anodizing process, in accordance with some described embodiments.

FIG. 3A shows part 300 after metal substrate 302 is anodized using a first anodizing process. Metal substrate 302 can be any suitable anodizable material, such as suitable aluminum, aluminum alloys, magnesium, magnesium alloys, and suitable combinations thereof. In some embodiments, metal substrate 302 is composed of a high performance (e.g., high mechanical strength) aluminum alloy, such as a 2000 series or a 7000 series aluminum alloy. In some embodiments, metal substrate 302 is composed of a 6000 series aluminum alloy, such as 6063 aluminum alloy. In some embodiments, metal substrate 302 is composed of a 4000 series aluminum alloy, such as a 4045 aluminum alloy. In some embodiments, the aluminum alloy includes at least 4.0% by weight of zinc (e.g., minimum zinc in some 7000 series aluminum alloys). In some applications, the aluminum alloy includes at least 5.4% by weight of zinc in order to achieve at least a desired substrate hardness. In some embodiments, the aluminum alloy includes at least 0.5% by weight of copper (e.g., minimum copper in some 2000 series aluminum alloys).

The first anodizing process converts a portion of metal substrate 302 to metal oxide coating 304, which include porous layer 301 and barrier layer 309. Porous layer 301 includes pores 306, which are formed during the anodizing process, and barrier layer 309 is generally free of pores 306 and is situated between metal substrate 302 and porous layer 301. Porous layer 301 and barrier layer 309 are both composed of metal oxide material 303, the specific composition of which depends on the composition of metal substrate 302. For example, an aluminum alloy metal substrate 302 can be converted to a corresponding aluminum oxide material 303.

In some cases where metal substrate 302 is composed of a high performance substrate, defects 307 can form within metal oxide coating 304. Defects 307 can be associated with certain alloying elements within metal substrate 302, such as copper in some 2000 series aluminum alloys and zinc in some 7000 series aluminum alloys. In some cases, defects 307 can include the alloying element(s) (e.g., zinc or copper). In some cases, defects 307 are in the form of cracks or voids within metal oxide coating 304. Defects 307 can be randomly distributed within metal oxide coating 304 and can in some cases connect with each other. As described above, defects 307 can act as pathways for corrosion inducing agents to reach metal substrate 302.

In some embodiments, the first anodizing process can produce a porous layer 301 that has a higher density of metal oxide material 303 compared to conventional anodizing processes. For example, pore walls 305 between pores 306 can be thicker than pore walls of a standard type II anodizing process. In particular, thickness 320 of pore walls 305 can range between about 10 nanometers and about 30 nanometers. Diameters 322 of pores 306 can range between about 10 nanometers and about 30 nanometers. In some embodiments, diameters 322 of pores 306 range between 10 nanometers to about 20 nanometers. In addition, thickness 312 of porous layer 301 can be relatively thick compared to conventional anodizing processes. For example, thickness 312 of porous layer 301 can ranges between about 6 micrometers and about 30 micrometers—in some embodiments ranges between about 10 micrometers and about 15 micrometers.

It should be noted that oxalic acid based anodizing can, in some cases, cause metal oxide coating 304 to have a yellow hue, sometimes associated with using an organic acid-based anodizing bath. This yellow color may be desirable or undesirable, depending on the application. For example, for exterior surfaces of consumer products, it may be desirable to have a yellow hue. In some cases, the yellow hue may be insignificant if the anodic oxide coating 304 is to be colorized by a dye or pigment. In other cases, it may be preferable to have a neutral color and undesirable to have a yellow hue. If a neutral color is desirable, the yellow hue can be offset using barrier layer thickening techniques, which will be described below with reference to FIG. 3B.

In some embodiments, the dense metal oxide coating 304 can be accomplished using a sulfuric acid based anodizing electrolyte. In particular embodiments, the electrolyte has a sulfuric acid concentration ranges between about 180 g/l and about 210 g/l. The temperature of the sulfuric acid based electrolyte ranges between about 10 degrees C. and about 22 degrees C. In anodizing voltage ranges between about 6 volts to about 20 volt, and the current density ranges between about 0.5 A/dm² and to about 2.0 A/dm². The anodizing process time can vary depending on a target thickness of metal oxide coating 304. In a particular embodiment, the anodizing time period ranges from about 10 minutes to about 100 minutes.

The higher density and thicker pore walls 305 of metal oxide coating 304 enhances the structural integrity of metal oxide coating 304 compared to conventional metal oxide coatings, despite the presence of defects 307. That is, defects 307 can be distributed within a more structurally dense metal oxide coating 304, thereby reducing the chance of defects 307 acting as entry points for corrosion inducing agents to reach metal substrate 302.

FIG. 3B shows part 300 after a second anodizing process is performed in order to further enhance the corrosion protection ability of metal oxide coating 304. The second anodizing process can promote anodic film growth without substantially promoting anodic film dissolution, thereby increasing the thickness 314 of barrier layer 309 to thickness t. This can be accomplished using a non-pore-forming electrolyte, such as one or more of Na₂B₄O₅(OH)₄.8H₂O (sodium borate or borax), H₃BO₃ (boric acid), C₄H₆O₆ (tartaric acid), (NH₄)₂.5B₂O₃.8H₂O (ammonium pentaborate octahydrate), (NH₄)₂B₄O₇.4H₂O (ammonium tetraborate tetrahydrate), and C₆H₁₀O₄ (hexanedioic acid or adipic acid). Suitable barrier layer thickening processes are described in detail in U.S. provisional application No. 62/249,079, filed Oct. 30, 2015, which is incorporated herein in its entirety.

Thickened barrier layer 309 can enhance the corrosion protection characteristics of metal oxide coating 304 by providing a thicker physical non-porous barrier between pores 306 and metal substrate 302. The anodizing process parameters can be chosen in order to provide a barrier layer 309 thickness t ranging from about 30 nanometers to about 500 nanometers. In some embodiments, thickness t of barrier layer 309 is chosen based on providing a color to metal oxide coating 304 by thin film interference coloring. It should be noted that the barrier layer thickening process can be performed without substantial change in the pore structure of metal oxide coating 304. That is, diameter 322 of pores can remain substantially the same before and after the barrier layer thickening process.

In some embodiments, metal oxide coating 304 is colorized by infusing a colorant, such as a dye, pigment or metal, within pores 306 and to impart a particular color to part 300. In some embodiments where metal oxide coating 304 has a colored hue from use of an oxalic acid or other organic acid electrolyte (e.g., from the first anodizing process), the colored hue combines with and enhances the color of the colorant. For example, a yellow hue caused by anodizing in an organic acid can combine with a red colorant to impart a darker or more orange aspect to metal oxide coating 304. Likewise, a yellow hue caused by anodizing in an organic acid can combine with a blue colorant to impart a green aspect to metal oxide coating 304. In this way, any suitable combination of color hues caused by anodizing in an organic acid and colorant can be used to impart a final color to metal oxide coating 304.

In addition to thickening barrier layer 309, anodizing in a non-pore-forming electrolyte can also smooth out the boundaries of barrier layer 309. For example, interface surface 316, which is defined by barrier layer 309 on one side and metal substrate 302 on another side, can have a smoother profile compared to the scalloped geometry prior to the barrier layer thickening process. The smoother and flatter interface surface 316 and/or pore terminuses 318 can increase the amount of visible light incident metal oxide coating 304 that is specularly reflected, thereby increasing the brightness of anodized part 300. Additionally or alternatively, the barrier layer smoothing process can flatten or smooth pore terminuses 318 of pores 306, such that flattened pore terminuses 318 can also specularly reflect incoming light. In this way, the smooth (i.e., flat) interface surface 316 and/or pore terminuses 318 can cause light to specularly reflect off interface surface 316 and/or pore terminuses 318, resulting in brightening the appearance of metal oxide coating 304.

In some embodiments, the barrier layer smoothing process is necessary in order to accomplish a particular level of lightness or a particular color, which can be measured using, for example, L*, a* and b* values as defined by CIE 1976 L*a*b* color space model standards. In general, L* indicates a level of lightness, with higher L* values associated with higher levels of lightness. Objects that reflect a yellow color will have a positive b* value and objects that reflect a blue color will have a negative b* value. Objects that reflect a magenta or red color will have a positive a* value and objects that reflect a green color will have a negative a* value. Some of these aspects are described in U.S. provisional application No. 62/249,079, filed Oct. 30, 2015, which is incorporated herein in its entirety.

The flatness or smoothness of interface surface 316 can be quantified as a profile variance defined by distance d between an adjacent peak and valley of the interface surface 316. Profile variance distance d can be measured, for example, from a transmission electron microscope (TEM) cross section image of the part 300. In some embodiments, interface surface 316 achieves a profile variance of no more than 5-6 nanometers.

FIG. 3C shows part 300 after colorant particles 311 are optionally deposited within pores 306 to give metal oxide coating 304 a desired color. Colorant particles 311 can be composed of any suitable colorant material, including suitable dye, pigment or metal material. In some embodiments, colorant particles 311 include black dye, such as Okuno Black 402 (manufactured by Okuno Chemical Industries Co., Ltd., based in Japan), in order to give metal oxide coating 304 a saturated black appearance. In some cases, a smoothed interface surface 316 can brighten the color provided by colorant particles 311. In some cases, thin film interference effects of barrier layer 309 create a color that combines with a color provided by colorant particles 311 to result in a final color. Additionally or alternatively, discoloration of metal oxide coating 304 by anodizing in an organic acid based electrolyte (e.g., oxalic acid) can combine with the a color provided by colorant particles 311 to result in a final color. That is, a final color of metal oxide coating 304 can be a result of one or more of the above-described brightening and color imparting techniques.

FIG. 3D shows part 300 after a pore sealing process is performed in order to effectively close pores 306, thereby enhancing the corrosion resistance, as well as chemical resistance, of metal oxide coating 304. In addition, the sealing process can make the outer surface of metal oxide coating 304 compatible for touching from a user, such as a user of an electronic device. Furthermore, if pores 306 include colorant particles 311, the sealing process can retain colorant particles 311 within metal oxide coating 304. The sealing process can hydrate the metal oxide material 303 of at least top portions of pore walls 305 of metal oxide coating 304. In particular, the sealing process can convert metal oxide material 303 to its hydrated form 334, thereby causing swelling of pore walls 305 and sealing of pores 306. The chemical nature of hydrated metal oxide material 334 will depend on the composition of metal oxide material 303. For example, aluminum oxide (Al₂O₃) can be hydrated during the sealing process to form boehmite or other hydrated forms of aluminum oxide. The amount of hydration and sealing can vary depending on the sealing process conditions. In some embodiments, only a top portion of pores 306 of metal oxide coating 304, while in some embodiments substantially the entire length of pores 306 of metal oxide coating 304 is sealed. Any suitable pore sealing process can be used, including exposing part 300 to hot aqueous solution or steam. In some cases, additives are added to the aqueous solution, such as nickel acetate or commercial additives, such as Okuno Chemical H298 (manufactured by Okuno Chemical Industries Co., Ltd., based in Japan).

After sealing, the metal oxide coating 304 can provide superior hardness and scratch resistance to part 300, as well as provide a desired cosmetic appearance to part 300. The relatively greater density of metal oxide material 303 makes metal oxide coating 304 harder and more chemically resistant than conventional anodic oxide coating, which can be useful in applications where metal oxide coating 304 corresponds to an exterior surface of a consumer product (e.g., devices of FIG. 1). In some embodiments, the metal oxide coating 304 on part 300 is characterized as having a hardness value of at least 200 HV.

Corrosion resistance of part 300 can be measured using standardized salt spray testing, such as per ASTM B117, ISO9227, JIS Z 2371 and ASTM G85 standards. In particular embodiments, part 300 has a salt spray test corrosion resistance measurement of about 336 hours using ASTM B117 standard salt spay techniques. Corrosion resistance can also be measured using ocean water testing, such as per ASTM D1141-98 standards. Examples showing improved corrosion resistance of samples having anodic films with thickened barrier layers tested under salt spray and ocean water procedures are described below with reference to FIGS. 8-12. This is a dramatic improvement in comparison to a part having a standard type II metal oxide coating (i.e., without barrier layer thickening). In some embodiments, the final thickness of metal oxide coating 304 (including thickness 312 and thickness t) is between about 6 micrometers and about 30 micrometers.

FIG. 4 shows flowchart 400, which indicates a process for forming a metal oxide coating in accordance with some embodiments. At 402, a substrate undergoes an optional surface pretreatment. In some embodiments, the surface pretreatment involves polishing a surface of the substrate to a mirror polish reflection. In some embodiments, the substrate surface is polished until the surface achieve a gloss value of 1500 gloss units or greater, as measured at 20 degree reflection. In a particular embodiment, the gloss value is about 1650 gloss units as measured at 20 degree reflection. The level of flatness/smoothness of the substrate surface prior to anodizing can be important in some embodiments in order to help achieve a sufficiently smooth barrier layer after a barrier layer thickening process is performed (see FIG. 3C). In other embodiments, the substrate undergoes a texturing process, such as an abrasive blasting and/or a chemical etching process, in order to form a blasted or matte texture to the substrate surface. Other surface pretreatment processes can include degreasing and de-smutting (e.g., exposure to a nitric acid solution for 1-3 minutes). Care should be taken on mirror polished surfaces, however, to assure the degreasing and de-smutting do not significantly damage the mirror polished surface of the substrate. The substrate can be composed of any suitable anodizable material, such as a suitable aluminum alloy.

At 404, a metal oxide coating is formed using a first anodizing process. In some cases, the first anodizing process involves using a first electrolyte that includes oxalic acid or sulfuric acid. In some embodiments, the first electrolyte includes a mixture of oxalic acid and sulfuric acid. In some embodiments, an electrolyte having sulfuric acid in a concentration between about 180 g/L and about 210 g/L held at a temperature between about 10 degrees C. and about 22 degrees C. using a current density between about 0.5 A/dm² and about 2.0 A/dm² was used to form a porous metal oxide coating having a thickness between about 6 micrometers and about 30 micrometers.

At 406, the barrier layer of the metal oxide coating is thickened using a second anodizing process, which can also be referred to as a barrier layer thickening process. The barrier layer thickening process can be performed in a non-pore forming electrolyte. In some embodiments, the non-pore forming electrolyte contains a non-pore forming agent, such as one or more of Na₂B₄O₅(OH)₄.8H₂O (sodium borate or borax), H₃BO₃ (boric acid), C₄H₆O₆ (tartaric acid), (NH₄)₂.5B₂O₃.8H₂O (ammonium pentaborate octahydrate), (NH₄)₂B₄O₇.4H₂O (ammonium tetraborate tetrahydrate), and C₆H₁₀O₄ (hexanedioic acid or adipic acid).

In some embodiments, the barrier layer thickening process involves anodizing in an electrolyte including a non-pore forming agent in a concentration of between about 10 g/L and 30 g/L held at an anodizing temperature of between about 8 degrees C. and 40 degrees C. for a time period of between about 1 minute to 2 minutes using a voltage between about 100 V and about 400 V. The voltage of the anodizing process can vary depending, in part, on a desired interference coloring provided by the barrier layer. In some embodiments, a voltage of between about 200 volts and about 500 volts, with low current density, is used. In a particular embodiment, a DC voltage is applied and increased at a rate of about 1 volt/second until a voltage of between about 200 volts and about 500 volts is achieved, which is maintained for about 5 minutes.

At 408, the metal oxide coating is optionally colored using any suitable coloring process. In some embodiments, dye, pigment, metal or a suitable combination thereof is deposited within pores of the metal oxide coating in order to achieve a desired color. At 410, the metal oxide coating is sealed to seal at least top portions of the pores within the metal oxide coating. This can increase the mechanical strength and corrosion resistance of the metal oxide coating.

FIGS. 5A and 5B show TEM cross section images of a part after a type II anodizing process and prior to a barrier layer thickening process. The part includes substrate 502, anodic oxide film 504 and barrier layer 506. Substrate 502 is composed of a 6063 aluminum alloy. Anodic oxide film 504 is formed from a sulfuric acid based (type II) anodizing process and has pore diameters ranging between about 10 nanometers to about 20 nanometers. The thickness of barrier layer 506 ranges between about 10 nanometer and about 20 nanometers.

FIGS. 6A and 6B show TEM cross section images of the part in FIGS. 5A and 5B after a barrier layer thickening process. In this example, a electrolyte having borax was used. The thickness of barrier layer 506 was increased to between about 60 nanometers and about 70 nanometers, and was found to provide good corrosion protection for substrate 502.

FIGS. 7A and 7B show SEM cross section images of anodic films prior to and after a barrier layer thickening process is performed. FIG. 7A shows part 700 after an anodizing process converts a portion of substrate 702 to metal oxide layer 704. Substrate 702 is composed of a 6063 aluminum alloy and was anodized using a sulfuric acid based (type II) anodizing process. The resulting metal oxide film layer 704 has pores 706 and a barrier layer 709 having a thickness between about 10 nm and about 20 nm. FIG. 7B shows part 700 after a barrier layer thickening process, and where barrier layer 709 is thickened to thickness 710 of between about 300 nm and about 500 nm.

Corrosion resistance evaluation of the anodic film having the thickened barrier layer can be determined using any suitable testing process. For example, the anodized substrate can be subjected to a salt-spray test or ocean water test and then inspected by eye and/or by color measurements to determine whether there is a color change. FIGS. 8-12 show aluminum alloy samples with and without a thickened barrier layer before and after a salt-spray test and an ocean water test, indicating the effectiveness of a thickened barrier layer for protecting an underlying substrate from corrosion. Prior to the salt-spray or ocean water testing, each sample in FIGS. 8-12 was dyed using a black dye (i.e., Okuno Black 402) to create a saturated black color in order to easily determine any color changes due to corrosion. After the salt-spray or ocean water testing, the color of each sample was visually evaluated and measured using standard CIE 1976 L*a*b* color space model measurements.

FIG. 8 shows perspective views of custom 6063 aluminum alloy samples 802, 804, 806 and 808 before and after a salt-spray testing procedure in accordance with ASTM B117 standard procedures for 336 hours. Sample 802 includes an type II anodic film without a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 804 includes an type II anodic film without a thickened barrier layer after being subjected to the salt-spray testing procedure. Sample 806 includes an type II anodic film with a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 808 includes an type II anodic film with a thickened barrier layer after being subjected to the salt-spray testing procedure. Table 1 below summarizes L*a*b* values before and after the salt-spray testing procedure.

TABLE 1 6063 Aluminum Alloy (Custom) Salt-Spray Testing Non-thickened barrier layer Thickened barrier layer L* a* b* L* a* b* Before 29.10 −0.52 −2.65 30.95 −.90 −3.48 After 19.22 −0.64 −5.24 30.02 −0.83 −3.13 Difference 9.88 0.12 2.59 0.93 −0.07 −0.35

As indicated by FIG. 8, the sample 804 without the thickened barrier layer is visibly much darker after the salt-spray testing compared to the sample 808 with the thickened barrier layer after the same salt-spray testing. Table 1 indicates a much larger difference in L* and b* values for the sample 804 without the thickened barrier layer compared to L* and b* values for the sample 808 with the thickened barrier layer.

FIG. 9 shows perspective views of market grade 6063 aluminum alloy samples 902, 904, 906 and 908 before and after a salt-spray testing procedure in accordance with ASTM B117 standard procedures for 336 hours. Sample 902 includes an type II anodic film without a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 904 includes an type II anodic film without a thickened barrier layer after being subjected to the salt-spray testing procedure. Sample 906 includes an type II anodic film with a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 908 includes an type II anodic film with a thickened barrier layer after being subjected to the salt-spray testing procedure. Table 2 below summarizes L*a*b* values of a market grade 6063 substrate samples before and after the salt-spray testing procedure.

TABLE 2 6063 Aluminum Alloy (Market Grade) Salt-Spray Testing Non-thickened barrier layer Thickened barrier layer L* a* b* L* a* b* Before 28.31 −0.75 −2.87 31.84 −1.24 −3.23 After 16.67 −1.69 −5.39 24.13 −1.98 −5.70 Difference 11.64 0.94 2.52 7.71 0.74 2.47

FIG. 9 indicates that sample 904 without the thickened barrier layer was visibly darker after the salt-spray testing compared to the sample 908 with the thickened barrier layer after the same salt-spray testing. Table 2 indicates a much larger difference in L* values for sample 904 without the thickened barrier layer compared to L* values for the sample 908 with the thickened barrier layer.

FIG. 10 shows perspective views of a 7000 series aluminum alloy samples 1002, 1004, 1006 and 1008 before and after a salt-spray testing procedure in accordance with ASTM B117 standard procedures for 336 hours. Sample 1002 includes an type II anodic film without a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 1004 includes an type II anodic film without a thickened barrier layer after being subjected to the salt-spray testing procedure. Sample 1006 includes an type II anodic film with a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 1008 includes an type II anodic film with a thickened barrier layer after being subjected to the salt-spray testing procedure. Table 3 below summarizes L*a*b* values of the 7000 series alloy substrate before and after the salt-spray testing procedure.

TABLE 3 7000 Series Aluminum Alloy Salt-Spray Testing Non-thickened barrier layer Thickened barrier layer L* a* b* L* a* b* Before 29.06 −0.61 −2.88 32.01 −1.11 −3.71 After 22.65 −1.06 −4.13 31.33 −1.07 −3.33 Difference 6.41 0.45 1.25 0.68 −0.04 −0.38

FIG. 10 indicates that sample 1004 without the thickened barrier layer was visibly darker after the salt-spray testing compared to the sample 1008 with the thickened barrier layer after the same salt-spray testing. Table 3 indicates a much larger difference in L* values for sample 1004 without the thickened barrier layer compared to L* values for the sample 1008 with the thickened barrier layer.

FIG. 11 shows perspective views of market grade 6063 aluminum alloy samples 1102, 1104, 1106 and 1108 before and after an ocean water testing procedure in accordance with ASTM D1141-98 standard testing procedures (without heave metal). The ocean water testing procedures in accordance with ASTM D1141-98, Formula a, Table x1.1, Section 6 was used. In particular, 42 grams of “sea salt” was mixed with 1 liter of deionized water, having a pH of 8.1 to 8.3. The samples were dipped for 168 hours.

Sample 1102 includes an type II anodic film without a thickened barrier layer that was not subjected to ocean water testing procedure. Sample 1104 includes an type II anodic film without a thickened barrier layer after being subjected to the ocean water testing procedure. Sample 1106 includes an type II anodic film with a thickened barrier layer that was not subjected to a ocean water testing procedure. Sample 1108 includes an type II anodic film with a thickened barrier layer after being subjected to the ocean water testing procedure. Table 4 below summarizes L*a*b* values of the market grade 6063 aluminum alloy substrate before and after the ocean water testing procedure.

TABLE 4 6063 Aluminum Alloy (Market Grade) Ocean Water Testing Non-thickened barrier layer Thickened barrier layer L* a* b* L* a* b* Before 28.42 −0.83 −3.12 31.04 −1.14 −3.46 After 18.55 −0.96 −3.79 28.13 −1.78 −2.57 Difference 9.87 0.13 0.67 2.91 0.64 −0.89

FIG. 11 indicates that sample 1104 without the thickened barrier layer was visibly darker after the ocean water testing compared to the sample 1108 with the thickened barrier layer after the same ocean water testing. Table 4 indicates a much larger difference in L* values for sample 1104 without the thickened barrier layer compared to L* values for the sample 1108 with the thickened barrier layer.

FIG. 12 shows perspective views of market grade 4045 aluminum alloy samples 1202, 1204, 1206 and 1208 before and after an ocean water testing procedure in accordance with the same ASTM D1141-98 standard testing procedures described above with reference to FIG. 11. Sample 1202 includes an type II anodic film without a thickened barrier layer that was not subjected to ocean water testing procedure. Sample 1202 includes an type II anodic film without a thickened barrier layer after being subjected to the ocean water testing procedure. Sample 1206 includes an type II anodic film with a thickened barrier layer that was not subjected to a ocean water testing procedure. Sample 1208 includes an type II anodic film with a thickened barrier layer after being subjected to the ocean water testing procedure. Table 5 below summarizes L*a*b* values of the market grade 4045 aluminum alloy substrate before and after the ocean water testing procedure.

TABLE 5 4045 Aluminum Alloy (Market Grade) Ocean Water Testing Non-thickened barrier layer Thickened barrier layer L* a* b* L* a* b* Before 30.87 0.33 −0.16 31.14 −0.01 −6.42 After 27.24 0.91 1.34 28.15 −0.21 −6.28 Difference 3.63 −0.58 −1.50 2.99 0.20 −0.14

FIG. 12 indicates that sample 1204 without the thickened barrier layer was visibly darker after the ocean water testing compared to the sample 1208 with the thickened barrier layer after the same ocean water testing. Table 5 indicates a much larger difference in L* values for sample 1204 without the thickened barrier layer compared to L* values for the sample 1208 with the thickened barrier layer.

Results from described above with reference to FIGS. 8-12 and Tables 1-5 indicate that the barrier layer thickening processes described herein can improve the corrosion resistance and discoloration of any of a number of types of aluminum alloy substrates. In some embodiments, the L* value of the anodic film changes by no more than 9 after a salt-spray test per ASTM B117 standards or after an ocean water test per ASTM D1141-98 standards.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. 

What is claimed is:
 1. A method of anodizing an aluminum alloy substrate, the method comprising: forming a metal oxide film on the aluminum alloy substrate by anodizing the aluminum alloy substrate in a first electrolyte, the metal oxide film including a porous layer and a barrier layer; and increasing a thickness layer of the barrier layer by anodizing the aluminum alloy substrate in a second electrolyte different than the first electrolyte, wherein a final thickness of barrier layer ranges between about 50 nanometers to about 500 nanometers, wherein the porous layer includes pores having diameters ranging between about 10 nanometers to about 30 nanometers.
 2. The method of claim 1, wherein a hardness of the metal oxide film on the aluminum alloy substrate is about 200 HV or greater.
 3. The method of claim 1, wherein the first electrolyte includes sulfuric acid, oxalic acid, or a mixture of sulfuric acid and oxalic acid.
 4. The method of claim 1, wherein the second electrolyte includes at least one of Na₂B₄O₅(OH)₄.8H₂O (sodium borate or borax), H₃BO₃ (boric acid), C₄H₆O₆ (tartaric acid), (NH₄)₂.5B₂O₃.8H₂O (ammonium pentaborate octahydrate), (NH₄)₂B₄O₇.4H₂O (ammonium tetraborate tetrahydrate), or C₆H₁₀O₄ (hexanedioic acid or adipic acid).
 5. The method of claim 1, wherein a thickness of the porous layer is between about 6 micrometers and about 30 micrometers.
 6. An anodized part, comprising: an aluminum alloy substrate; and an anodic film disposed on the aluminum alloy substrate, the anodic film including: an exterior oxide layer having an outer surface corresponding to an outer surface of the anodized part, wherein the exterior oxide layer includes pores having diameters ranging between about 10 nanometers to about 30 nanometers, and a barrier layer positioned between the exterior oxide layer and the aluminum alloy substrate, wherein a thickness of the barrier layer ranges between about 50 nanometers to 500 about nanometers.
 7. The anodized part of claim 6, wherein the pores have diameters ranging between about 10 nanometers to about 20 nanometers.
 8. The anodized part of claim 6, wherein the pores are defined by pore walls having thicknesses ranging between about 10 nanometers to about 30 nanometers.
 9. The anodized part of claim 6, wherein the aluminum alloy substrate includes a 7000 series aluminum alloy or a 2000 series aluminum alloy.
 10. The anodized part of claim 6, wherein the aluminum alloy substrate includes at least 4.0% by weight of zinc and at least 0.5% by weight of copper.
 11. The anodized part of claim 6, wherein a thickness of the exterior oxide layer ranges between about 6 micrometers to about 30 micrometers.
 12. The anodized part of claim 6, wherein the anodic film has a hardness value of about 200 HV or greater.
 13. The anodized part of claim 6, wherein the anodic film has a black dye incorporated therein, wherein an L* value of the anodic film changes by no more than 9 after a salt-spray test per ASTM B117 standards or after an ocean water test per ASTM D1141-98 standards.
 14. An enclosure for an electronic device, the enclosure comprising: an aluminum alloy substrate having at least 4.0% by weight of zinc; and an anodic coating disposed on the aluminum alloy substrate, the anodic coating including: an exterior oxide layer having sealed pores having diameters ranging between about 10 nanometers to about 30 nanometers, and a barrier layer positioned between the exterior oxide layer and the aluminum alloy substrate, wherein a thickness of the barrier layer ranges between about 30 nanometers to about 500 nanometers.
 15. The enclosure of claim 14, wherein a thickness of the exterior oxide layer ranges between about 6 micrometers to about 30 micrometers.
 16. The enclosure of claim 14, wherein the anodic coating as measured from an exterior surface of the anodic coating has a hardness value of about 200 HV or greater.
 17. The enclosure of claim 14, wherein the pores have diameters ranging from about 10 nanometers and about 20 nanometers.
 18. The enclosure of claim 14, wherein the pores are defined by pore walls having thicknesses ranging from about 10 nanometers to about 30 nanometers.
 19. The enclosure of claim 14, wherein the aluminum alloy substrate has at least 5.4% by weight of zinc.
 20. The enclosure of claim 14, wherein the aluminum alloy substrate includes at least 0.5% by weight of copper. 